In vehicles driven by endless crawler tracks such as surface mining shovels, excavators, bulldozers and tanks, the crawler tracks include a series of links or shoes which are pivotally pinned together and have shoe lugs which are engaged and pushed by a rotating tumbler drive mechanism. The tumbler drive mechanism is mounted on a rotating shaft and has a sprocket-like shape including tumbler lugs which extend radially outward and engage the shoe lugs as the tumbler rotates to drive the shoes and thereby the entire crawler track. The crawler track and the tumbler are both subject to substantial wear which creates problems of proper fit between the tumbler lugs and shoe lugs and periodic replacement of the shoes, the tumbler and the shaft on which the tumbler is mounted. Since the shaft on which the tumbler is mounted, the tumbler, and the shoes all have different wear lives, additional complexity is involved in attempting to obtain maximum use out of the various components. The tumbler is subject to what might be considered normal wear due to abrasive engagement of the tumbler lugs with the shoe lugs and possible eventual fracture of the tumbler lugs. A second type of wear on the tumbler lugs is caused by wear of the crawler track resulting in an increase in the pitch, i.e., center to center distance at the connecting pins, of the shoes. Due to this pitch growth, the shoe lugs will interfere with the tumbler lugs and accelerate the wear on both types of lugs. Since there are fewer tumbler lugs, the damage to them is the most severe. Consequently, the tumbler has a shorter wear life than the crawler track and typically requires replacement twice as often as the shoe lugs. Because of the weight of the tumbler, its replacement necessitates the use of a crane which is expensive and increases the down time of the vehicle. Further, since the shaft on which the tumbler is mounted is supported in anti-friction bearings and the tumbler is very tightly mounted on the shaft, both the tumbler and the shaft must be removed in order to change the tumbler. In replacing the shaft and tumbler, there is risk of damaging the inboard anti-friction bearings of the shaft since the repositioning of the shaft must be done in a blind fashion.